The Fabry-Perot is known type of optical filter or interferometer structure that relies on an interference of multiple reflected beams. Incident light undergoes multiple reflections between two oppositely opposed coated surfaces which define therebetween an optical cavity. Each transmitted wavefront experiences an even number of reflections. When there is no phase difference between these wavefronts interference between them produces a transmission maximum. This occurs when the optical path difference is an integral number of whole wavelengths. At other wavelengths, destructive interference of the transmitted wavefronts reduces the transmitted intensity towards zero.
The transmission peaks can be made very sharp by increasing the reflectivity of the mirror surfaces. The ratio of the full width at half maximum (FWHM) of a transmission peak to the separation of successive peaks is termed the finesse. High reflectance results in a high finesse.
In an article entitled "Double-Cavity Electrooptic Fabry Perot Tunable Filter", Applied Optics, Sep. 1, 1982, Vol. 21, No. 17 William Gunning describes a development of an infrared multiple cavity electro-optically tuned Fabry-Perot fiber using LiNbO.sub.3. This filter, designed for operation in the 3-5 micron spectral band, was demonstrated by angle scanning using a He-Ne laser at 3.39 microns. A multiple-cavity configuration increased a free-spectral range of a 2-Angstrom bandwidth filter from 67 Angstroms to 670 Angstroms.
One significant application for a Fabry-Perot filter is in the construction of a Laser Receiver (LR). Traditional LR technology utilizes temporal-coherence discrimination to detect laser threats. This is accomplished with various types of etalons and detectors. As used herein an etalon includes two parallel, semi-reflecting surfaces, separated a finite distance apart. The space between the surfaces of an etalon can be filled by any optically transmitting material. The surfaces of the etalon are parallel and flat to at least (wavelength/20). Laser light incident on an etalon either interferes constructively or destructively. A laser detection is generated when there is a difference of signal levels from one etalon step size to another. Broad-band spectral sources transmit equally through all etalons and therefore are discriminated from laser radiation. This configuration may be employed for short-pulse lasers but is not sufficient to detect long-pulse or cw lasers.
Temporal coherence discrimination can also be achieved by using a single etalon in which the Optical Path Length (OPL) is modulated rapidly over the period of the pulse.
In this case, laser light generates a sinusoidal signal at the frequency of modulation. However, broadband light does not generate a sinusoidal signal since the average transmission of the etalon for broadband light remains substantially constant.
OPL modulation has previously been accomplished by mounting etalon plates between piezoelectric stacks. The OPL is varied by changing the distance between the etalon plates. However, these systems are sensitive to thermal vibrations and modulate at a relatively slow rate. Since the modulation rate is directly proportional to an ability to detect shorter pulses a lower limit is placed on the pulse width that can be discriminated or on the time required to declare a laser threat.
A second OPL modulation approach has employed the electro-optic effect. In this case the OPL is varied by varying the index of refraction of an electro-optic material. An etalon of electro-optic material is coated with dielectric coatings and a large voltage potential is applied to change the index of refraction via the electro-optic effect. The change in the index of refraction for light propagating along an optic axis of the material is given by the expression EQU n=-1/2n.sub.o.sup.3 r.sub.13 E.sub.3,
where n.sub.o is the refractive index, r.sub.13 is the electro-optic coefficient, and E.sub.3 is the applied field. Modulation of one half of a free spectral range (FSR) is required for maximum sensitivity. By example, to modulate half a FSR at a wavelength of 0.6328 microns requires a high voltage of approximately .+-.2 kV. This required high voltage necessitates using dielectric electrodes which are difficult to modulate at high frequencies. This type of device was constructed for use in a static mode as described by W. Gunning and P. Yeh, "Multiple-Cavity Infrared Electro-Optic Tunable Filter", SPIE Vol. 202, Active Optical Devices (1979), pg. 21. See also U.S. Pat. No. 4,269,481, May 26, 1981, "Multiple-Cavity Electro-Optic Tunable Filter" by P. A. Yeh and J. M. Tracy.
Other prior art of interest includes the following. In U.S. Pat. No. 4,793,675, Dec. 27, 1988 Y. Handa discloses a LiNbO.sub.3 optical waveguide said to employ a surface acoustic wave. In U.S. Pat. No. 4,184,738, Jan. 22, 1980, S. Wright discloses an optical waveguide having interdigitated or triangular electrodes and operating by Dec. 27, 1988, Risk discloses a fiber optic amplitude modulator employing surface acoustic waves that impact on the fiber. In U.S. Pat. No. 3,932,373, Dec. 2, 1975, Dabby et al. and in U.S. Pat. No. 4,396,246, Aug. 2, 1983 Holman discloses LiNbO.sub.3 waveguide modulators. In U.S. Pat. No. 3,874,782, Apr. 1, 1975 Schmidt discloses an LiNbO.sub.3 strip waveguide modulator having electrodes 16 on opposite sides of the crystal. And, in U.S. Pat. No. 3,877,781, Apr. 15, 1975 Kaminow discloses an LiNbO.sub.3 optical waveguide that employs three electrodes to modulate light passing therethrough.
However, none of this prior art discloses, and it is thus an object of the invention to provide, a Fabry Perot optical device that operates in accordance with an acousto-optic principle to modulate a light beam passing therethrough.
It is a further object of the invention to provide a Laser Receiver that includes an acousto-optic Fabry Perot optical device to detect the presence of a laser beam and which distinguishes a single line or multiple line laser source from a broadband source.